PHILADELPHIA--Pope Francis confronted the sexual abuse of children by Roman Catholic clergy on the final day of his first U.S. visit on Sunday, meeting with five adults abused as children and vowing to hold accountable those involved in the crimes and cover-ups. The pope left the United States on Sunday night, departing from Philadelphia International Airport on an American Airlines flight to Rome. He was seen off by a number of dignitaries, including Vice President Joe Biden. Wrapping up the six-day U.S. trip, the 78-year-old Argentine pope celebrated Mass outside the Philadelphia Museum of Art, a neoclassical edifice astride a rocky hill overlooking the Schuylkill River, before the largest crowd of his visit. Philadelphia Archbishop Charles Chaput said the crowd numbered more than a million. "I have in my heart the stories of suffering and the pain of the minors who were sexually abused by priests," the pope, speaking in his native Spanish, told bishops in Philadelphia after meeting privately with the sexual abuse victims, three women and two men. "This disgrace keeps burdening me, that the people who had the responsibility of caring for these tender ones raped them and caused them great pain. God weeps for the sexual abuse of children," the leader of the world's 1.2 billion-member Catholic Church added. The pope offered his most comprehensive comments on the sexual abuse scandal in his 2-1/2 year papacy and used his strongest language yet in condemning it and promising that "all responsible will be held accountable." It was believed to be the first time any pope has publicly described the abuse as rape. The pope was criticized by abuse victims earlier in his U.S. trip after he addressed the scandal on Wednesday. At that time, he did not utter the words "sexual abuse," praised bishops for their "courage" and "generous commitment to bring healing to victims" and lamented that "pain" from the scandal had burdened them. On Sunday, Francis told the victims he deeply regretted that some bishops had failed to protect children and was disturbed that in some cases bishops were the abusers. The meeting marked just the second time Francis has met victims of clergy sexual abuse and was the first on a foreign tour. He previously met with victims at the Vatican. "I am profoundly sorry that your innocence was violated by those who you trusted," the pope told the victims, according to a transcript provided by the United States Conference of Catholic Bishops. "For those who were abused by a member of the clergy, I am deeply sorry for the times when you or your family spoke out to report the abuse, but you were not heard or believed. Please know that the Holy Father heard you and believes you." Vatican spokesman Federico Lombardi said "two or three" of five victims were abused by priests or Catholic educators, with the others abused by a family member or non-Catholic teacher. Reports that priests had sexually abused children and bishops had covered up their actions emerged in 2002 in the United States and in other countries several years later. The scandal damaged the Catholic Church worldwide, eroding its moral authority and requiring costly legal settlements. During the Mass, Francis told the throngs along Philadelphia's Benjamin Franklin Parkway the world must be united in facing challenges including climate change and lifting people out of poverty. "The urgent challenge of protecting our home includes the concern of bringing all of the human family together in the pursuit of sustainable and integral development," he said. The pope earlier met with 71 prisoners, 60 men and 11 women in blue prison garb, who were among the approximately 2,800 inmates at Curran Fromhold Correctional Facility, a drab compound in Philadelphia's northeastern outskirts with barbed wire topping its outer walls. The pope, an outspoken opponent of the death penalty and lengthy prison terms, sat in a wooden chair fashioned by the inmates and told them that "confinement is not the same thing as exclusion" from society. He then walked around to speak individually with inmates, placing his hand on their foreheads, shaking hands and hugging some who stood to greet him. The pope's comments on the abuse scandal drew a mixed reaction from victims and their advocates. David Clohessy, who heads the Survivors Network of Abuse by Priests and was sexually assaulted by a priest as a child, said, "Every pope follows an arc. First no talk, then vague talk, then clearer talk, then tougher talk, but it remains just talk." Anne Barrett Doyle, co-director of BishopAccountability.org, a private group tracking the sexual abuse scandal, said the test of the pope's resolve will be whether he follows through on his promise to hold the culprits accountable. Mark Rozzi, a Pennsylvania state legislator who was raped as a young teen by his parish priest, said the pope had delivered"powerful words. "Now we have to put those words into real action," Rozzi said. As many as 100,000 U.S. children may have been the victims of clerical sex abuse, insurance experts said in a paper presented at a Vatican conference in 2012. The Archdiocese of Philadelphia has been the subject of multiple damaging grand jury reports relating to the scandal, which by the Church's own estimate has involved 6,400 clergy credibly accused between 1950 and 2013 nationwide. Twelve U.S. dioceses have filed for bankruptcy in part due to hefty settlements, more than $3 billion nationwide, paid to victims.